This invention relates generally to semiconductor RF amplifiers, and more particularly the invention relates to a CMOS semiconductor amplifier which has improved operating efficiency.
Cascaded multistage amplifiers are used to create power gain for a small (e.g., 200 mV) high frequency RF signal. Class A amplifiers, which draw current continuously, are needed for small input signals but are inefficient in operation. Class AB amplifiers do not draw current continuously but require larger input signals for efficient operation. Thus, a class A amplifier stage has typically been used with small signals to provide larger signals to following class AB amplifier stages.
The present invention is directed to providing a more efficient small signal amplifier by using only Class AB amplifier stages.
In accordance with the invention a CMOS RF amplifier is provided for amplifying a small input signal by first providing DC bias voltages to the CMOS transistor gates, and capacitively coupling a small input signal to the voltage biased gates. In effect, the small input signal becomes a larger gate voltage in driving the RF amplifier in an AB operation mode.
In a preferred embodiment, the DC bias voltages are taken from a P transistor which is approximately identical to the P transistor of the CMOS transistor pair, the P transistor being connected to a current source for generating a gate voltage that is resistively coupled to the gate of the P channel transistor of the CMOS transistor pair. Similarly, a N channel transistor is connected with a current source for generating a gate voltage which is resistively coupled to the gate of the N transistor of the CMOS transistor pair. The N channel transistors are approximately identical in structure.
The invention and objects and features thereof will be more readily apparent from the following detailed description and dependent claims when taken with the drawings.